Effects of acute and chronic exercise on myocardial ultrastructure.

Histological and ultrastructural evaluation of the heart reveals that structural alterations may be induced by both acute and chronic exposure to exhaustive exercise. Following an acute bout of exercise to the point of exhaustion, the normal heart does not appear hypoxic as assessed by mitochondrial morphology. However, small shifts in intracellular volume fractions as well as dilatation of transverse tubules and sarcoplasmic reticulum have been observed. The functional significance of these ultrastructural changes is not well understood, but could explain any alterations in contractility observed at this point in time. Following repeated exposure to strenuous dynamic exercise on a daily basis for an extended period of time (endurance training), the effects seem to be principally on the vasculature of the heart, with no alterations in composition of the myocardial cell. This is manifested by an increase in size of the major coronary arteries and a corresponding change in the number of capillaries. The chronic effects of exercise on coronary arterioles are unknown. The results of retrospective studies in humans and laboratory experiments with primates and lower mammals indicate that these training-induced structural alterations may play a protective role with respect to coronary heart disease. Most importantly, this role would appear to be both preventive and rehabilitative in nature.